The present invention relates generally to MR imaging systems, and, more particularly, to a system and method for fast and efficient data transfer of MR signal data between an MR scanner and a data processing or image reconstruction unit. The invention includes the use of optical modulation to encode data onto an optical carrier for transmission, such as over a fiber-optic cable or other optical link.
MR systems operate by detecting the free induction decay (FID) signals emitted by excited tissues. When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The scanning equipment of the MR system, such as the transmit/receive RF coils and gradient coils, is often located in an MR scanner within a magnetically shielded room. The set of received NMR signals resulting from a scan sequence are digitized and sent to a data processing unit, often located outside the shielded room, for image reconstruction using one of many well known reconstruction techniques. It is desirable that the imaging process, from data acquisition to reconstruction, be performed as quickly as possible for improved patient comfort and throughput. Furthermore, data for reconstruction should contain as high a signal to noise ratio as possible to reduce the likelihood that additional scans will be needed.
One particular factor lending to the speed of an imaging process is the signal to noise ratio of the link between the MR scanning equipment and the data processing unit. A system which achieves a high signal to noise ratio (SNR) for accurate data receipt will have a faster and improved imaging process.
Most MR systems utilize electrical signals to convey data from the MR coils to the image reconstruction unit. These systems typically use coaxial cable, or other copper-wire based media, to transfer data. However, electrical signals traveling through such cables can experience undesirable amounts of noise and signal power loss. The connections of these cables are often bulky, to accommodate for impedance-modifying circuits, filters, amplifiers, and other electronics required for electrical communication. Furthermore, electrical data transmission over cables can suffer from data loss or noisy SNR from such phenomena as EMI and crosstalk. Conventionally, MR systems aim at having less than 1 dB of noise figure on signal transmission. And, while many accommodations have been made to mitigate the impact of these limitations, such as high performance amplifiers to improve SNR, the limitations are compounded by the introduction of large coil arrays.
Some recent MR systems have explored the use of optical data transfer. These systems rely upon lithium-niobate optical modulators to encode data onto light energy from high power, low noise laser sources. Generally, lithium-niobate is used as a controllable refractive crystal, to modulate laser light energy by interfering two light beams from the same laser source in which one beam or both beams are phase modulated. These modulators are also rather bulky (to accommodate the crystal), are expensive, and are not easily integrated or retrofitted into existing scanners. A related drawback is the difficulty associated with connecting and disconnecting lithium-niobate modulators. Since these modulators must individually be integrated into an MR coil housing, an optical connection for the carrier must be engaged and disengaged, as opposed to an electrical connection as in coax-based systems. In addition, MR systems using lithium-niobate modulators are less efficient, usually requiring high power laser carriers and upwards of 3 volts to modulate the phase of a signal by Π radians.
It would therefore be desirable to have a system which achieves optical MR data transmission and maintains high SNR, while remaining compact, inexpensive, and benefiting from lower power carrier requirements. In addition, it would be beneficial if it was easily retrofitted and did not require direct optical disconnection and reconnection for each new patient for improved reliability and easier servicing.